Lithium metal has many uses and, to name a few, they include nuclear power application where a blanket of the liquid metal or its molten salts is used for breeding purposes in nuclear fusion reactors, in lightweight, compact lithium/sulfur batteries for electric cars and for power plant load leveling purposes, as a degasifier in the production of high-conductivity copper and bronze, and in the synthesis of compounds for use in the field of medicine.
Lithium metal is generally produced by electrolysis of an eutectic mixture of highly pure molten lithium chloride and potassium chloride.
There are naturally occurring brines in the United States which contain reasonable concentrations of lithium, in the form of the chloride, so as to be considered viable reserves for lithium recovery. Three particular sources include Searle's Lake, California, the Great Salt Lake, Utah, and Clayton Valley, Nevada. The last is the most economical source of lithium since the magnesium to lithium ratio is low, generally about 1.15:1, which allows for a simplified process of concentrating, purifying and recovering lithium chloride brine. Lithium carbonate is then obtained by treatment of the brine with soda ash.
To make lithium metal, the lithium carbonate is converted to lithium hydroxide via a liming process, and the latter compound in turn is converted to lithium chloride by treatment with hydrochloric acid followed by drying. This is a very circuitous and expensive route to lithium chloride, since lithium originally exists as the chloride in the natural brine. Thus for many years there has been the need for a direct economical method for recovering lithium chloride as such from natural brines.
In lithium containing brines, the magnesium, calcium, sulfate and boron content have to be minimized in order to produce a lithium chloride product suitable for production of lithium metal by eletrolysis. Alkaline earth metals, such as magnesium and calcium, must be removed from the lithium chloride, otherwise they will be present as contaminants in the lithium metal. During electrolysis of lithium chloride, non-volatile anions, such as sulfate and borate, will build up, resulting in rapid short-circuiting of the cell.
The presence of boron also results in losses of lithium on concentration of the brine. More particularly, on concentration of a lithium containing brine, boron, if present in significant quantities, e.g. 0.2% or more, will precipitate lithium values as lithium tetraborate tetrahydrate (Li.sub.2 B.sub.4 0.sub.7.4H.sub.2 0). Depending upon the boron concentration of the brine, such losses of lithium can be economically signficant.
Heretofore boron has been removed from, or the concentration thereof has been substantially reduced in lithium chloride brines by various methods including treatment of a brine with slaked lime to precipitate calcium borate and, in the case of brines containing substantial magnesium impurities, magnesium borate; by absorbing borates on clays, on HCO.sub.3.sup.- and C1 type resins, or on activated alumina in the presence of magnesium; by precipitating borate as a boro-phosphate concentrate by treating the brine with lime in combination with phosphoric acid; by acidification of the brine to precipitate boric acid, and by solvent-solvent extraction, i.e. with n-butanol.
It is a primary object of this invention to provide an economical process for removing or at least substantially reducing the amount of boron as well as magnesium and sulfate impurities in a lithium containing brine.
Another object of this invention is a novel process for treating a lithium containing brine to reduce losses of lithium in the form of compounds of boron on further concentration of the brine.
Still another object of this invention is to provide a lithium containing brine which may be further concentrated to produce lithium chloride suitable for use in the electrolytic production of lithium metal.
These and other objects of this invention will become apparent from a consideration of this specification and appended claims.